The idea of a callable unit was initially conceived by [[John Mauchly]] and [[Kathleen Antonelli]] during their work on [[ENIAC]],<ref name="Dasgupta2014">{{cite book|author=Subrata Dasgupta|title=It Began with Babbage: The Genesis of Computer Science|url=https://books.google.com/books?id=tXBVAgAAQBAJ&pg=PT155|date=7 January 2014|publisher=Oxford University Press|isbn=978-0-19-930943-6|pages=155–}}</ref> and recorded in a January 1947 Harvard symposium on "Preparation of Problems for [[EDVAC]]-type Machines".<ref name=mauchly0>{{cite book|first=J.W.|last=Mauchly|chapter=Preparation of Problems for EDVAC-Type Machines |author-link=John Mauchly|editor-first=Brian|editor-last=Randell|title=The Origins of Digital Computers|doi=10.1007/978-3-642-61812-3_31|publisher=Springer|date=1982|pages=393–397 |isbn=978-3-642-61814-7 }}</ref> [[Maurice Wilkes]], [[David Wheeler (British computer scientist)|David Wheeler]], and [[Stanley Gill]] are generally credited with the formal invention of this concept, which they termed a ''closed sub-routine'',<ref>{{Cite conference |last1 = Wheeler |first1 = D. J. |author-link1 = David Wheeler (computer scientist) |chapter = The use of sub-routines in programmes |doi = 10.1145/609784.609816 |title = Proceedings of the 1952 ACM national meeting (Pittsburgh) on - ACM '52 |pages = 235 |year = 1952 |chapter-url = http://www.laputan.org/pub/papers/wheeler.pdf|doi-access = free }}</ref><ref>{{cite book |last1= Wilkes |first1= M. V. |last2= Wheeler |first2= D. J. |last3= Gill |first3=S. |title= Preparation of Programs for an Electronic Digital Computer |publisher= Addison-Wesley |year= 1951}}</ref> contrasted with an ''open subroutine'' or [[Macro (computer science)|macro]].<ref>{{cite encyclopedia |last=Dainith |first=John |title="open subroutine." A Dictionary of Computing | date=2004 |url=http://www.encyclopedia.com/doc/1O11-opensubroutine.html |encyclopedia=Encyclopedia.com |access-date=January 14, 2013}}</ref> However, [[Alan Turing]] had discussed subroutines in a paper of 1945 on design proposals for the NPL [[Automatic Computing Engine|ACE]], going so far as to invent the concept of a [[Call stack|return address stack]].<ref>{{Citation | last = Turing | first =Alan M. | author-link = Alan Turing | title = Report by Dr. A.M. Turing on proposals for the development of an Automatic Computing Engine (ACE): Submitted to the Executive Committee of the NPL in February 1946 | year = 1945 }} reprinted in {{Cite book | editor-last = Copeland | editor-first = B. J. | editor-link = Jack Copeland | title = Alan Turing's Automatic Computing Engine | ___location = Oxford | publisher = Oxford University Press | publication-date = 2005 | isbn = 0-19-856593-3 | year = 2005 | url-access = registration | url = https://archive.org/details/alanturingsautom0000unse |page=383}}</ref>
The idea of a subroutine was worked out after computing machines had already existed for some time. The arithmetic and conditional jump instructions were planned ahead of time and have changed relatively little, but the special instructions used for procedure calls have changed greatly over the years. The earliest computers and microprocessors, such as the [[Manchester Baby]] and the [[RCA 1802]], did not have a single subroutine call instruction. Subroutines could be implemented, but they required programmers to use the call sequence—a series of instructions—at each [[call site]].
